1. Field of the Invention
The present invention relates to a lock-up control system for directly coupling input and output elements of a torque converter normally coupled with an output side of a prime mover of an automotive vehicle and forming a part of a power train.
2. Description of the Related Art
A power transmission unit, such as an automatic power transmission, includes a torque converter in a power transmission system for amplifying engine output torque and for absorbing torque fluctuation. The torque converter is generally low in power transmission efficiency since it transmits the engine torque between an input element and an output element via a working fluid. Therefore, recently, torque converters which establish direct engagement between the input elements and the output elements by means of lock-up clutches under vehicle traveling condition, in which the torque amplification function and torque fluctuation absorbing function are not required, are increasing.
On the other hand, upon establishing direct engagement between the input element and the output element of the torque converter by means of the lock-up clutch, when direct engagement (lock-up) is progressed abruptly, a lock-up shock is inherent associating with relative rotation between the input element and the output element of the torque converter. Accordingly, upon progressing engagement of the lock-up clutch, it is typical to gradually vary engaging condition at predetermined time gradient by feedforward controlling or feedback controlling control command value. Then, upon termination of the control, since completion of lock-up is difficult to judge, the control is terminated when the lock-up control command value reaches a maximum value.
In such lock-up control, as a result of releasing of accelerator pedal for the prime mover, when a vehicle traveling condition enters into a lock-up region for directly coupling the input element and the output element of the torque converter, lock-up control is performed in the similar manner as discussed with reference to FIG. 6.
At a timing t.sub.1 of FIG. 6 it is assumed that, as a result of completely releasing of the accelerator pedal to make a throttle valve open angle TVO to 0, the vehicle traveling condition enters into a lock-up range to establish direct engagement between the input element and the output element of the torque converter. Then, a lock-up solenoid actuation duty cycle (lock-up duty) D as the lock-up control command value is gradually increased at a predetermined time gradient from 0%. In response to this, a torque converter release pressure P.sub.R is gradually decreased. As a result, an engaging capacity of the lock-up clutch determined by a pressure difference between the release pressure P.sub.R and a torque converter apply pressure P.sub.A, is gradually increased. An engaging capacity is continued to increase even after a moment t.sub.2 where the engaging capacity of the lock-up clutch reaches a coasting capacity required for coasting of the vehicle, engagement of the lock-up clutch is gradually progressed. By this, lock-up condition of the torque converter can be established without causing a shock. Then, at a moment t.sub.3 where a lock-up duty D becomes a maximum value, the lock-up control is terminated smoothly. Then, by setting the lock-up duty D at the coasting capacity, control is transited to a coasting capacity control for maintaining the coasting capacity condition to maintain a lock-up condition suitable for coasting of the vehicle.
However, is such lock-up control upon releasing of the accelerator, the engaging capacity of the lock-up clutch can become excessive between the moment t.sub.2 where the engaging capacity becomes the coasting capacity required for coasting of the vehicle and the moment t.sub.3 where the lock-up duty D becomes the maximum value. Therefore, it is inherent to cause degradation of response characteristics in releasing lock-up. Accordingly, when an abrupt braking is performed during a period between the moments t.sub.2 and t.sub.3, particularly when vehicular wheels tends to cause locking, releasing of lock-up by releasing of the lock-up clutch cannot be effected quick enough to cause stalling of the engine.